Radiographic diagnosis of an expansile bone lesion in a dog

An old dog had an expansile lesion affecting the ulnar diaphysis. The lesion had clinical and radiographic features typical of a bone cyst; however, computed X-ray tomography indicated that the lesion had a tissue content incompatible with a true cyst. The histological diagnosis was osteosarcoma. This report emphasises the highly variable radiological appearance of canine osteosarcoma; biopsy is required to establish the diagnosis because the radiological signs may mimic a lesion of different aetiology.